Spray coating apparatus

ABSTRACT

AN ELECTROSTATIC, COMPRESSED-AIR SPRAY GUN WHEREIN THE LIQUID TO BE ATOMIZED IS FORMED ON A CONDUCTIVE SUPPORT INTO A FILM HAVING AN EXTENDED EDGE COINCIDENT WITH ONE EDGE OF A NARROW OPENING THROUGH WHICH A HIGH-VELOCITY STREAM OF ATOMIZING AIR IS DISCHARGED. THE OPPOSITE EDGE OF SUCH OPENING IS DEFINED BY AN ELECTRODE BETWEEN WHICH AND THE FILM-SUPPORT THERE IS MAINTAINED AN ELECTRICAL POTENTIAL DIFFERENCE. THE GUN MAY BE PROVIDED WITH AN INSULATED SPRAY-SURROUNDING ELECTRODE WHICH IS MAINTAINED AT AN EFFECTIVE SPRAY-REPELLING POTENTIAL BY THE TRANSMISSION OF ION-BORNE CHARGES TO OR FROM IT.

U 21', 1971 W, FERRANT 3,606,972

SPRAY COATING APPARATUS Original Filed April 21, 1967 I 3 Sheets-Sheet l INVENTOR Fig. 2 WOLFGANG FERRANT Sept. 21; 1971 w, FERANT 3,606,972

SPRAY COATING APPARATUS Ozfiginal Filed April 21, 1967 3 Sheets-Sheet 2 N IO J' f!) a; t if I 1 x3? E 1 w v --N INVENTOR WOLFGANG FERRANT Sept. 21, 1971 w. FERRANT SPRAY COATING APPARATUS 3 Sheets-$heet I5 Original Filed April 21, 1967 will 4 INVENTOR WOLFGANG FERRANT United States Patent ()1 ice 3,606,972 SPRAY COATING APPARATUS Wolfgang Ferraut, Bozen, Italy, assignor to Ransburg Electro-Coating Corp., Indianapolis, Ind. Continuation of application Ser. No. 632,583, Apr. 21, 1967. This application May 12, 1969, Ser. No. 826,071 Claims priority, application Germany, Apr. 28, 1966, F 49,044 Int. Cl. B05b 5/00; F23d 11/28 U.S. Cl. 239-15 15 Claims ABSTRACT OF THE DISCLOSURE An electrostatic, compressed-air spray gun wherein the liquid to be atomized is formed on a conductive support into a film having an extended edge coincident with one edge of a narrow opening through which a high-velocity stream of atomizing air is discharged. The opposite edge of such opening is defined by an electrode between which and the film-support there is maintained an electrical potential difference. The gun may be provided with an insulated spray-surrounding electrode which is maintained at an effective spray-repelling potential by,,the transmission of ion-borne charges to or from it.

This is a continuation of application Ser. No. 632,583, filed Apr. 21, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a device for forming a spray of electrically charged liquid particles with the aid of a high-velocity air stream. The device is intended primarily for use in the electrostatic application of sprayed liquid to surfaces in order to form a coating thereon.

Electrostatic spray-coating systems employing compressed air to effect atomization differ in respect to the manner in which the spray particles become charged and also in respect to whether or not they provide an electrostatic field which extends to the surface being coated and which aids in promoting the electrostatic deposition of the charged spray particles on such surface. Some systems, for example as shown in U.S. Pat. 1,855,869 to Pugh, contemplate charging the coating material by conduction before it is atomized; while others, for example as shown in U.S. Pat. 2,247,963 to Ransburg et al., first atomize the coating material and then charge the atomized particles by ion bombardment. Some systems, like those of the aforesaid Pugh and Ransburg patents, employ depositing fields extending to the surface being coated; in others, for example those of U.S. Pat. 2,890,388 to Croskey et al. and U.S. Pat. 2,894,175 to Lamm, the field provided is largely confined within the spraying device itself and has no significant depositing effect.

The mobility of a charged particlei.e., its response to electrostatic forces-varies inversely as the mass of the particle and directly as the product of the charge on the particle and the strength of the field in which it finds itself. A high charge-to-mass ratio in particles intended to respond to electrostatic forces is therefore a desideraturn. In spray guns using compressed air for atomization, charging of the liquid by conduction prior to its atomization, as by applying a high voltage to a conventional spray gun, has not resulted in a high charge-to-mass ratio in the resulting spray particles, probably due at least in part to the initial formation .of relatively large particles which, after separation from the fed stream of charged liquid, are broken up into finer particles. The electrical capacity of a liquid droplet varies directly as its diameter; and when a charged liquid particle breaks up into a plurality of smaller particles the aggregate capacity of the smaller particles is greater than the capacity of the initial large particle, but the aggregate charge on the smaller particles is no greater than that on the initial particle, so that even if the initial particle was fully charged, the smaller particles will be incompletely charged.

Because mere charging of a compressed air spray gun results in incomplete charging of the ultimate spray particles, it is common practice to provide means for charging those particles by bombardment with atmospheric ions produced at a discharge electrode. Whether such an electrode is carried by the gun or located remote from the gun, and whether or not it is arranged to maintain a depositing field, commercial spray-coating systems relying on charging by ion bombardment require the use of relatively high electrical potentials, from a few kilovolts in systems where the charging field is confined within the spraying device up to or kilovolts or more in systemswhere the field extends to the surface being coated. Such high voltages create safety and other problems, and voltage sources necessary to create them are relatively bulky and expensive. Further, in the case of hand-held spray guns to which a high-voltage source is connected, the stiffness and weight of the relatively large cables necessary to convey high voltage complicates manipulation of the gun.

SUMMARY OF THE INVENTION An electrostatic spray gun embodying this invention charges the liquid by conduction before it is atomized, employs a high-velocity air stream for atomizing, and is so constructed as to limit to a small size the initially formed particles and to charge those particles effectively with the use of relatively low voltages. A preferred form of spray gun embodying the invention has an air passage terminating in a narrow annular discharge opening defined by two concentric electroconductive elements. The inner one of such elements is so constructed that liquid supplied to it near its center is spread into a thin film having a circular leading edge substantially coincident with the inner edge of said annular discharge opening. An electrical potential difference, desirably as close as practicable to that which would cause sparking, is maintained between the two elements defining the annular opening, with the result that a high potential gradient exists at the circular edge of the liquid film. Air discharged at high velocity through the opening atomizes charged particles from the film-edge and forms them into a spray which can be directed toward an article or surface maintained at a particle-attracting potential. Since the spray particles are detached from the liquid film at a point of high gradient, they will bear high charges. The charged spray particles emerging from the gun exert a space charge effect creating an electrostatic field extending to the article or surface being coated and aiding in the electrostatic deposition of particles closer to such article or surface.

The electrostatic depositing effect can be increased by providing the gun with an insulated (electrically floating), spray-surrounded electrode maintained at sprayrepelling potential either by impressing it with ion-transmitted charges of the same sign as those possessed by the spray particles or by causing the emission from it of charges of opposite sign. For example, a stream of ionized air charged similarly to, but independent of, the spray can be directed into contact with the electrode; the electrode can be provided interiorly with sharp dis charge elements providing for the escape from the electrode of charges opposite in sign to the charges on the spray particles; or means may be provided for ionizing the air in the region within the electrode to an extent such that some of the charge on the spray particles passing through that region will be bled off to the electrode. Any of such expedients makes it possible, without the necessity for using voltages higher than that imposed across the annular discharge opening, to maintain the spray-surrounding electrode at a potential such as will promote deposition of the spray on a surface to be coated.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration, largely diagrammatic in char acter, illustrating an electrostatic spraying system embodying a hand-held spray gun;

FIG. 2 is a side elevation on an enlarged scale of the front end of the barrel shown in FIG. 1 with parts broken away and shown in section to illustrate the interior construction;

FIG. 3 is a side elevation in partial section of a spray gun equipped with a spray-surrounding electrode and embodying means for impressing a charge on such electrode by the use of an ionized air stream;

FIG. 4 is a view similar to FIG. 3, but showing a modified electrode and electrode-charging means;

FIG. 5 is a fragmental section, on the enlarged scale the line 5--5 of FIG. 4; and

FIG. 6 is a view similar to FIG. 5 illustrating a modified construction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The system shown in FIG. 1 comprises a hand-held spray gun designated in its entirety by the reference numeral 10 and including a barrel 11 and a handle 12. Conduits 13 and 14 extending from the lower end of the handle 12 respectively connect the gun to a source 15 of air under pressure and a source 16 of liquid to be sprayed. A voltage lead 17 extends into the handle 12 from one terminal of a voltage source 18, the other terminal of which is grounded as indicated at 19. A trigger 20 mounted on the gun controls the supply of air, liquid, and voltage in any convenient manner. Exterior portions of the gun 10, including the barrel 11, handle 12, and trigger 20 are grounded as through a conductive sheave surrounding, but insulated from, the voltage lead 17. PIG. 1 also shows a grounded article 21 in position to receive spray from the gun.

The barrel of the gun, as shown in FIG. 2, comprises a tubular housing 23 provided at its front end with an inwardly projecting radial flange 24 having a central opening 25- through which the spray emerges. Within the casing 23, and conveniently coaxial therewith, are concentric outer and inner tubes 26 and 27, the tube 26 being connected to the air line 13 and the tube 27 to the liquid line 14. Mounted on the front end of the tube 26, through the medium of a ring 28 of insulating material, is an annular electrode 29. At their front ends, the tube 27 is enlarged in external diameter and the electrode 29 reduced in internal diameter to leave a narrow annular discharge opening 30 through which the spray emerges for passage through the central opening 25 at the end of the barrel-housing 23. Overlying the enlarged front face of the tube 27 is a cap 32 conveniently supported by an integral stem 33 from within the tube 27. The cap 32 is spaced a slight distance forwardly from the end of the tube 27 to leave at the periphery of the cap a narrow gap through which liquid emerging from the tube 27 can escape in the form of a film having a circular edge substantially coincident with the circular edges of the front face of the tube 27 and of the cap 32.

The inner tube 27, and desirably also the outer tube 26, are electrically connected to the barrel-housing 23 to be grounded as above indicated. The electrode 29 is connected, preferably through a current-limiting resistor 34, to the voltage lead 17. Voltage conveyed to the electrode 29 through the lead 17 and resistor 34 creates an electric field extending across the annular discharge opening 30 to the grounded tube 27 and cap 32. With voltage applied to the electrode 29, and with compressed air and liquid supplied to the tubes 26 and 27 respectively, the liquid is formed into a thin film between the end face of the tube 27 and cap 32, and air emerging at high velocity through the opening 30 atomizes liquid from the circular edge of such film. Since the annular opening 30 is narrow, having a width equal to only a small fraction of an inch, a strong field can be created between the edge of the film and the electrode 29 with the use of a. relatively low voltage, say on the order of a few hundred volts. As a result of being formed in a region of high field strength, the liquid particles discharged from the gun are highly charged.

It is to be noted that the size of particles formed by the high velocity air emerging through the opening 30 is limited by the thickness of the liquid film between the cap 32 and the end face of tube 27. Feeding the liquid to the atomizing site in the form of a thin film having an extended edge has advantages over atomizing the liquid from a solid stream such as would emerge from a circular orifice. One such advantage results from the fact that, at the same rate of liquid discharge, the thickness of the film will be substantially less than the diameter of a stream emerging from a circular orifice, with the result of inhibiting the presence of relatively large liquid particles in the discharged spray. Since the size of the particles as initially formed is limited by the narrow gap between the cap 32 and the face of tube 27, and since the particles are separated from the charged liquid film in a region of great field strength, the spray particles will have the high charge-to-mass ratio previously indicated as desirable.

As compared with the conventional compressed-air spray gun, the atomizing arrangement above described provides a further advantage in that it allows the energy of the air stream to be more efficiently imparted to the liquid in the effecting of atomization. Given the same rate of air discharge, directing a confined air stream across the edge of a thin film increases the proportion of fine particles in the spray above that attained when the air is directed against the sides of a comparatively thick stream, as in a conventional gun. This feature is of benefit even in a non-electrostatic gun.

Charged spray particles projected from the opening 30 and through the opening 25 in the barrel-housing 23, are attracted to a grounded article being coated. At the same time, mutual repulsion of the similarly charged articles tends to spread the spray, a result which is frequently undesirable. To counteract such spray-spreading tendency in those cases where it is not desired, I may provide the wall of the housing 23, at spaced points about its circumference, with a plurality of air-admitting openings 35. The aspirating action of the discharged spray induces a flow of air through the openings 35 and radially inwardly between the flange 24 and the front face of the electrode 29. Such inwardly directed air flow impinges on the spray and tends to prevent it from spreading under the influence of the electrostatic forces acting between the spray particles. To prevent contact with electrode 29 through openings 35, a grounded barrier 22 may be supported within tubular housing 23 between openings 35 and the electrode 29.

The dense cloud of spray particles adjacent the gun exerts a space charge effect which creates between it and the grounded article being coated an electrostatic field promoting the deposition of particles nearer the article. This effect can be supplemented by the action of a spray-surrounding driving electrode mounted on the front end of the gun, as shown in FIG. 3. A spray-re pelling potential applied to the driving electrode will create between it and the grounded article a field promoting the deposition of charged spray particles on the article and also serving, like the above mentioned inwardly directed air flow, to reduce spreading of the spray as a result of mutual repulsion acting between spray particles. The effectiveness of the driving electrode in promoting deposition and limiting spreading of the spray will depend upon its potential. Most desirably, the driving electrode has a potential above that of the spray passing through it. While the driving electrode might be connected to a voltage source providing such a higher potential, I prefer for reasons apparent above to avoid the use of such a high potential source. That result can be accomplished by insulating the driving electrode from the gun and supplying it with an ion-transmitted charge by means now to be described.

In FIG. 3, the driving electrode 37 is shown as mounted on the gun through the medium of a cup-like member 38 of insulating material. The electrode is hollow-walled, and the space between the walls communicates with a tube 40 of insulating material through which a stream of ionized air is directed. To aid in transmitting electrical charge from such ionized air stream to the electrode, the space between the walls map be filled with some porous material, preferably conductive, such as metal shavings or metal wool 41. Since the electrode is insulated from the gun, and also from ground, the charge which accumulates from the ionized air stream builds up and raises the potential of the electrode until the leakage from it equals the rate at which it receives charge from the ionized air stream.

The air to be ionized may be taken from the air supplied to the gun from the conduit 14 and directed into a valved tube 42 mounted on the barrel-housing 23 exteriorly thereof. The tube 42 communicates with a metalwalled ionizing chamber 43 the walls of which are grounded, conveniently through the tube 42 or other support connected to the barrel-housing 23. Mounted within the ionizing chamber 43 through the medium of a perforated insulator 44 is a pointed ionizing electrode 45 which is connected to an appropriate voltage source. The chamber 43 has a small outlet opening communicating with the tube 40, and the tip of the electrode 45 is mounted as closely as practicable to that outlet opening. Air flowing through the tube 42 and the openings in the insulator 44 becomes charged as it passes the tip of the electrode 45 and flows through the tube 40 into the interior of the driving electrode 37, the walls of which are porous or perforated to the extent necessary to permit the air to escape after surrendering its charge to the electrode. To minimize the leakage of charges from the ionized air stream, it is desirable that the interior of the tube 40 be of small dimension so that the air velocity through it will be high. The work which must be done in moving the gaseous ions from the region of relatively low potential adjacent the discharge electrode 45 to the region of higher potential at the electrode 37 is derived from the kinetic energy of the air stream flowing through the tubes 42 and 40.

The cup-like insulator supporting the driving electrode 37 may be provided in its wall with openings 46 for the admission of air serving the same purpose as the air admitted to the barrel-housing 23 through the openings 35 therein. Additional openings 47 for the same purpose may be provided in the electrode 37.

Another manner of charging an insulated, spray-surrounding driving electrode without the necessity of using a voltage source other than that employed to create the field across the annular opening 30 utilizes the charges on the spray particles passing through the electrode. Assuming that the charges borne by the spray particles are negative in sign, as is preferred, the negative space charge of the dense particle-cloud will repel negative charges in and on the electrode to its outer surface and attract position charges to its inner' surface. By permitting the latter charges to escape from the electrode,

the electrode can acquire a net negative charge. One arthe positive charges to escape to the spray. The net negative charge on the electrode builds up to increase the electrode-potential until, as in the case of the device of FIG. 3, the rate of charging equals the rate of leakage.

Charging the electrode in the manner just indicated involves decreasing the electrical charge on the spray, for the positive charges acquired by the spray from the electrode neutralize some of the negative charge initially borne by the spray; and the net effect is the same as if some of the negative charge on the spray had been bled off to the electrode. However, as explained below, the rate at which the spray loses charge to the electrode can be controlled, it is possible to build up on the electrode a potential high enough so that the strength of the depositing field it creates will more than compensate for the loss of charge suffered by the spray.

The rate of transmission of charge between the spray and the electrode can readily be varied by controlling the ionizing effect of the devices '50, for example in the manner illustrated in FIG. 5. As there shown, each device 50 is screw-threadedly mounted in a conductive sleeve 51 which is in turn screw-threadedly mounted in an opening 52 in the wall of the electrode. By varying the position of the device 50 radially of the electrode, and/or by varying the extent to which its tip is shielded by the sleeve 51, the extent to which the atmosphere inside the electrode is ionized and rendered conductive can be controlled.

Another electrode-charging means employing the charge on the spray is illustrated in FIG. 6. That means comprises a body 55 of radio-active material capable of emitting positive ions, such as alpha particles, which will ionize the atmosphere traversed by the spray as it passes through the electrode. The body 55 is supported on the inner side of a plug 56 screw-threadedly mounted in an opening 57 in the wall of the electrode so that the posi tion of the body 55 radially of the electrode can be varied. When positioned near the periphery of the electrode, the body 55 is relatively ineffective to ionize the atmosphere inside the electrode, but is effectiveness will increase as it is moved radially inwardly.

The ionizing devices 50 or 55 should be so adjusted as to obtain a maximum electrostatic depositing effect; and as such depositing effect depends on both the magnitude of the charge on the spray particles and the potential of the driving electrode, the depositing effect can suffer by transmitting either too much or too little charge from the spray particles to the driving electrode.

In considering the manner in which the driving electrodes of FIGS. 3 and 4 are charged, the electrode and the atmosphere inside it can be regarded as an electrical free body, conductive and insulated from ground. Electrical charge is continuously forced into such free body by the kinetic energy of the atomizing air stream. Part of the charge so supplied leaves on the projected spray and the remainder is transmitted, directly or in effect, through the conductive atmosphere to the electrode serving to raise the potential thereof until the rate at which the electrode acquires charge is balanced by the loss of charge by leakage, as through the insulator supporting the electrode.

I claim as my invention:

1. An electrostatic spray device comprising first and second adjacent elements cooperating to provide a narrow opening therebetween, at least a portion of the second element being electrically conductive,

means including a body of liquid having an edge from which particles are atomized in the opening on the first element and spaced from the second element, means including a voltage source connected to the conductive portion of the second element and to the body of liquid to establish a potential difference between the conductive portion of the second element and the edge of the body of liquid and thereby create an electric field across the opening between the conductive portion of the second elementand the edge of liquid to electrically charge the edge of liquid, and

means directing a high velocity stream of air over the edge of the body of liquid and through the opening to atomize the liquid and project it from the opening as a spray of charged liquid particles. 2. A spray gun according to claim 1 wherein the conductive portion and the liquid body are connected to opposite terminals of the voltage source. 3. The electrostatic spray gun of claim 1, wherein the first and second elements are substantially concentric thereby providing an annular air passage adjacent the Opening which is progressively decreasing in cross sectional area.

4. An electrostatic spray gun comprising a confined atomizing-air passage adapted to be con nected to a source of air under pressure and terminating in an extended narrow opening said opening being defined by first and second electroconductive elements electrically insulated from each other,

means for forming liquid supplied to said gun into a thin film in contact with said first element and having and extended edge which is substantially conincident with the adjacent edge of said opening,

means for connecting said elements to a voltage source to establish a potential difference between said elements and thereby create an electric field across said opening to charge said liquid film, and

means for directing a high-velocity stream of atomizing air across the extended film-edge and outwardly through said opening carrying said liquid from the gun as a spray of charged liquid particles.

5. .An electrostatic spray gun according to claim 4 wherein the opening is annular and said second element surrounds said first element.

6. An electrostatic spray gun according to claim 5 wherein a generally cylindrical housing surrounds the second element and is spaced radially outwardly therefrom to provide an air passage having inlet openings in the wall of the housing, said housing having at its forward end an inwardly projecting, spray-surrounding flange spaced forwardly from said elements for directing toward the axis of the spray atmospheric air aspirated into the passage through the housing openings.

7. An electrostatic spray gun according to claim 4 with the addition of a spray-surrounding driving electrode insulated from the gun, and means for supplying the electrode with an electrical charge to impart to it a sprayrepelling potential.

8. An electrostatic hand spray gun having a barrel comprising,

inner and outer elements defining an annular atomizing air pasage terminating in an annular discharge opening at the front end of the barrel, at least the front end portions of the elements at the discharge opening being electroconductive and insulated from each other,

means for feeding liquid as a thin film to one edge of said discharge opening, and

means for producing a difference in electrical potential between said front end portions to create an electric field extending across said opening to charge said liquid film whereby atomizing air passing through said passage and opening will atomize liquid from the edge of said film and project it forwardly from the opening as a spray of charged particles.

9. A spray gun according to claim 8 wherein said liquid feeding means is in said inner element and presents said film to the inner edge of the discharge opening.

10. A spray gun according to claim "8 wherein except for the front end portion of one of said elements, said elements are grounded, said front end portion of the one element being insulated from the remainder of such element and connected to one terminal of a voltage source the other terminal of which is grounded.

11. A spray gun according to claim 10 wherein it is the outer element whose front end portion is connected to the voltage source, said gun including a grounded annular housing surrounding and spaced from such front end portion and having at its front end an inwardly extending annular flange defining an opening for passage of the discharged spray, said housing having an air inlet opening in rear of said flange, whereby the discharged atomizing air, by aspiration, induces through said housing flow of an air stream directed inwardly against said spray to oppose its expansion.

12. An electrostatic hand spray gun comprising a barrel having grounded electrically conductive elements defining an atomizing-air passage and forming the exterior of the barrel;

an electrically conductive element at the front end portion of the atomizing air passage within the grounded electrically conductive elements, insulated from and cooperating with at least one of the grounded electrically conducting elements at the front end portion of the barrel to form a narrow opening; means for feeding liquid as a narrow stream in contact with one of the elements forming the edge of the opening, and means connected with the electrically conductive elements that form the opening to create a difference of electrical potential across the narrow opening and I charge the liquid stream whereby atomizing-air passing through the passage and opening will atomize liquid from said stream and project it forwardly from the opening as a spray of charged particles.

13. An electrostatic spray gun comprising first and a second means forming an atomizing and charging device,

at least a portion of one of said means being electroconductive,

one of said means forming liquid to be atomized into a thin film having an edge, the edge of liquid being at a potential different from the potential of the electroconductive portion of said means, the electroconductive portion of said means cooperating with the edge of liquid to create an electric field with a high potential gradient at the edge of liquid, and means including an opening adjacent the film directing a high velocity stream of air over the edge of liquid to atomize the liquid into a spray of charged liquid particles and project the spray from the spray gun. 14. The electrostatic spray gun of claim 13 including a spray surrounding electrode cooperatively associated with the gun and means supplying the electrode with an electrical charge to impart to it a spray-repelling potential. 15. The electrostatic gun of claim 14 including means having an air inlet opening adjacent the first and second means whereby the flow of atomizing air, by aspiration, induces through said inlet opening an air flow directed inwardly against said spray to oppose its expansion.

References Cited UNITED STATES PATENTS 3,000,574 9/1961 Sedlacsik 23915 3,049,092 8/ 1962 Sedlacsik et al 23915 3,111,266 11/1963 Axelson et al "239-15 3,317,138 5/1967 Fraser 239-15 FOREIGN PATENTS 1,358,199 3/1964 France 23915 1,038,865 8/1966 Great Britain 239-15 697,378 11/1965 Italy 239l5 LLOYD L. KING, Primary Examiner 

